The invention relates to a communication system, a master unit, a user unit, a method of operating a communication system and a method of operating a user unit.
In production and automation technology, serial bus systems are increasingly used in which the remotely arranged devices of machine peripherals such as I/O modules, transducers, drives, valves and operator terminals communicate with automation, engineering or visual display systems via an efficient real-time communication system. In this arrangement, all users are networked together via a serial bus, preferably via a field bus, the data exchange via the bus being carried out, as a rule, on the basis of the master-slave principle.
The active bus users on the bus system, the control devices, as a rule, possess a bus access authorization and determine the data transfer on the bus. The active bus users are called the master units in the serial bus system. In contrast, passive bus users are, as a rule, machine peripheral devices. They do not receive a bus access authorization, i.e. they are only allowed to acknowledge received information signals or transfer information signals to a master unit on request by the latter. Passive bus users are called slave units in the serial bus system.
To avoid complex cabling, field bus systems having a master-slave structure are generally arranged in ring topology, all bus users being coupled to a ring-shaped transmission path. An information signal generated by the master unit is fed into the ring-shaped transmission path by the master unit and successively passes through the slave units serially coupled to the ring-shaped transmission path and is then received again by the master unit and evaluated. Master-slave systems can also be designed as multi-master systems.
As a rule, the information signals are organized by the master unit into data packets which are composed of control data and useful data, preferably using the Ethernet standard which provides for data packets having a length of up to 1500 bytes with a transmission speed which, at the same time, is high at 100 Mbit/sec. Each of the slave units coupled to the ring-shaped transmission path exchanges the useful data intended for it with the Ethernet message when the Ethernet message fed in by the master unit passes through on the ring-shaped transmission path.
As a rule, the master-slave communication systems with ring structure are configured in such a manner that the master unit has a transmitting unit as data injection point and a receiving unit as data extraction point into a transmission medium. The individual slave units are then coupled together on the transmission path to form a chain, wherein each user is coupled to two neighbors and the first and last user in the chain is coupled to the master unit. The data packets are transmitted in one direction starting from the master unit via its transmitting unit to the first slave unit coupled and from there to the next one, until the last slave unit in the chain is reached, and then back to the receiving unit of the master unit. Each slave unit has, for receiving the circulating data packets from the previous user, an interface with a receiving unit and, for forwarding to the following user, an interface with a transmitting unit, a processing unit being arranged between receiving and transmitting unit in order to process the data packets passing through the slave unit, i.e. to exchange the useful data allocated to the slave unit with the data packets. The ring-shaped communication system with master-slave structure is often designed in such a manner that the master unit forms a physical line with the slave units arranged at it, the transmission medium having a dual-line structure and each slave unit having two ports with a combined transmitting/receiving unit, transmitting and receiving unit being short-circuited in the output port of the last slave unit in the transmission chain. The data packets injected into the first line by the master unit via its receiving unit are processed by the slave units on the forward path and are then simply forwarded only to the receiving unit of the master unit on the return path via the second line.
A central requirement for master-slave communication systems, particularly when they are used in production and process automation, is a high fault tolerance, that is to say the capability of the communication system to maintain the required function, i.e., for example, the production of a workpiece, in spite of the occurrence of faults. In this context, faults in the communication system which must be overcome without impairment of the process are, in addition to faults in the data packets, also the failure of entire transmission links, in particular, for example due to physical separation of the transmission medium.
To achieve a fault-tolerant master-slave communication system, particularly in the case of link faults, i.e. in the case of the failure of entire transmission sections, dual-ring structures operating in opposite directions are frequently used. Thus, a communication system having a master-slave structure in which the master unit is serially coupled to a multiplicity of slave units via two communication paths operating in opposite directions, is described in U.S. Pat. No. 4,663,748, wherein the master unit simultaneously sends out the data packets over two communication paths. The slave unit then has two processing units which are in each case coupled between the two communication paths in order to process the data messages passing through. Furthermore, coupling units which can be activated are arranged in the users, so that when a link fault occurs, e.g. a break in a communication line, it reconfigures the communication system by monitoring the signals on both transmission rings and correspondingly switching over the communication system, in such a manner that a failure due to the link fault of a greater section of the communication system or even a total failure is prevented.
In DE 103 12 907 A1, it is also proposed to arrange the slave unit in such a manner that on each communication path in the direction of data transmission, first a processing unit and then a multiplexer having two inputs and one output is arranged. The multiplexer is coupled with its inputs in each case to the two processing units of the slave unit and coupled with its output to the associated communication path. In fault-free normal operation, each of the two multiplexers switches through the processing unit arranged on the associated communication path. In fault mode, when a link fault occurs on the associated communication path, however, the processing unit on the other communication path is then switched through. This design of the slave unit enables the communication system to be reconfigured essentially in real time in the fault case.
However, fault-tolerant master-slave communication systems having a dual-ring structure, in which the individual slave units in each case have two processing units for processing the data message passing through, provide for high hardware and switching complexity of the slave units and thus increase the cost. Furthermore, each slave unit must decide in normal operation which of the two data packets passing through the two processing units should be used for device control which greatly restricts the use of such communication systems at the required high data transmission rates. In addition, the known fault-tolerant communication systems with dual-ring topology require that the master unit responds separately to a link fault and switches from normal operation into fault operating mode.